The present invention relates generally to a technique for improving the transmission of high bit rates or high signal frequencies within a substrate, and more particularly, to a technique for minimizing losses, skin effect, and cross-talk between adjacent signal conductors, and increasing propagation speed.
The present state of the art in multi-layer substrates is to sandwich a conventional rectangular stripline between two substrate layers, which are generally dielectric layers.
Each substrate layer has an associated loss tangent, which indicates the amount of signal loss due to the dielectric effects of the substrate layers that are adjacent to the stripline. At high bit rates or high signal frequencies, the effect of the loss tangent increases, thereby causing more loss. An additional problem inherent in the prior art results from a phenomenon known as xe2x80x9cskin effectxe2x80x9d. When current is passed through a conductor, the current tends toward the outside of the conductor thereby creating an outer skin of current. The cross-sectional area of the skin is less than the cross-sectional area of the conductor, thereby creating additional losses. The skin effect worsens as the frequency of the transmitted signal increases as shown by the following equation:
skin depth=1/(fxcfx80"sgr"xcexc)xe2x80x83xe2x80x83(1)
where f is equal to signal frequency, "sgr" is equal to the electrical conductivity of the conductor or signal track and xcexc is equal to the permeability of the conductor or signal track. Accordingly, higher data rates lead to smaller skin depth, which in turn leads to higher losses.
An additional problem in the prior art has been the existence of crosstalk, which is a category of noise induced primarily by the electromagnetic coupling between signal lines. In printed wiring boards, crosstalk can occur by the electrical coupling between nearby conductors in a given layer. Crosstalk increases with longer track coupling distances and smaller separation between tracks. Furthermore, crosstalk becomes a greater problem at higher frequencies. The problems associated with crosstalk are fully discussed in U.S. patent application Ser. No. 09/443,128, filed Nov. 18, 1999, now U.S. Pat. No. 6,444,922, issued Sep. 3, 2002, which is incorporated by reference herein in its entirety. The aforementioned application discloses the use of Embedded shielded stripline (ESS) technology.
In view of the foregoing, it would be desirable to provide a technique for improving signal reach and signal integrity during signal transmission at high bit rates or high signal frequencies which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to minimize the losses due to non-uniform current distribution and high dielectric loss tangents. Accordingly, it would be desirable to provide a technique for manufacturing a multi-layer substrate for improving signal reach and signal integrity in an efficient and cost effective manner.
According to the present invention, a technique for facilitating signal transmission at high signal frequencies is provided. In one embodiment, the technique is realized by providing a multi-layer substrate. The multi-layer substrate comprises a conductor or pair of conductors, a first dielectric layer on a first side of the conductor or pair of conductors, and a second dielectric layer on a second side of the conductor or pair of conductors. An air channel is formed in the first dielectric layer, the air channel formed to be substantially coextensive with the conductor or pair of conductors. An electrically conductive shield surrounds the conductor or pair of conductors, the first dielectric layer, and the second dielectric layer.
In accordance with other aspects of the present invention, a multi-layer substrate comprises an elongated conductor or pair of conductors having a continuous main path and discrete spaced edges protruding from opposing edges of the continuous main path. A first dielectric layer is provided on a first side of the conductor or pair of conductors, the first dielectric layer having an air channel substantially coextensive with the continuous main path and a solid portion overlapping with the discrete spaced edges. A second dielectric layer is provided on a second side of the conductor or pair of conductors, the second dielectric layer having an air channel substantially coextensive with the continuous main path of the conductor or pair of conductors and a solid portion overlapping with the discrete spaced edges. An electrically conductive shield surrounds the conductor or pair of conductors and the first and second dielectric layers.
In accordance with further aspects of the present invention, a supporting layer is provided adjacent to at least one of the dielectric layers. The supporting layer is provided on an opposite side of the dielectric layer from the conductor or pair of conductors.
In accordance with still further aspects of the present invention, the width of the conductor or pair of conductors is increased in order to provide impedance matching in view of the low dielectric constant of the air channel.
In accordance with another aspect of the invention, a method is provided for creating embedded shielded stripline structures with air channels. The method comprises forming an air channel within a dielectric layer and placing additional layers on each side of the dielectric layer. The additional layers including at least a top layer, a bottom layer, and at least one signal layer adjacent the air channel. The method further comprises laminating the top layer, bottom layer, dielectric layer, and signal layer and forming trenches from the top layer to the bottom layer and through the dielectric layer and signal layer. Finally, the method comprises applying metallization to the trenches.